Rapid and accurate acoustic measurement systems and methods for determination of individual noise sound contributions from a plurality of physical noise sources at a target or reference location is of great interest to various industries such as the automotive industry. These acoustic measurement systems and methods are highly useful for example to identify and eliminate sources of cabin noise at the driver's or passenger's ear position in cars, trucks, busses and other motorized vehicles. The determined noise sound contributions of particular physical noise sources such as motor, gearbox, tyres, exhaust pipe etc. can be systematically and individually attenuated, or even amplified, to reach a desired target noise sound characteristic at the target location. In other situations, these acoustic measurement systems and methods are highly useful to determine individual noise sound contributions from physical noise sources of the motorized vehicle at a target location which is external to the motorized vehicle for example in the far field. This determination is of significant interest in numerous types of applications for example to evaluate and reduce acoustic noise emissions of a broad range of motorized vehicles like aeroplanes, cars, trucks, trains, busses, lawn mowers etc. However, there typically exists many different independent noise sound sources in an operational condition of the motorized vehicle and the total noise sound pressure at the target position will be a complex mixture of contributions by many different physical noise sources along respective sound/vibration transfer paths.
In the prior art, so-called reciprocal or reverse acoustic transfer path based methodologies for determination of the sources of cabin noise at a target position have been applied. These transfer path methodologies, often designated as source-path-contribution, comprises the placement of one or several near-field indicator microphones close to each significant noise source of the motorized vehicle for recording of microphone signals during operation of the motorized vehicle. Thereafter, acoustic transfer functions are measured between an assumed source position or positions of each noise source and all the mounted indicator microphones at the other noise sources. By combining operating and transfer function data it is possible to at least partially eliminate the spill-over or cross-talk contributions from other noise sources to the microphone signal(s) measured at any given noise source. The separation of the different noise sources is the first step in this approach and the next step is to propagate each noise source to the target location e.g. at a certain point inside the motorized vehicle, to assess the contribution form the noise source. This known methodology is prone in its ability to accurately separate the different noise sources because a number of simplifying assumptions are made. One of these simplifying assumptions is that each noise source is modelled by a single point source or possibly by a set of assumed point sources. However, real noise sound sources often possess a distributed character due to source dimensions like an engine or exhaust system. This makes it difficult to find representative positions of the indicator microphones. Experience also shows that the measured noise contributions are quite sensitive to the exact positioning of the indicator microphones around a distributed noise source. Hence, the point source assumption can lead to significant errors in the measured transfer functions and the estimated noise source contributions at the target location. Another problem associated with these source-path-contribution methodologies is that data must be acquired during an operational condition of the motorized vehicle and transfer function measurements must be made during a stationary condition of the motorized vehicle. This is a time consuming process which is further worsened by the fact that practical volume velocity sources for the transfer function measurements are unable to cover the entire frequency range of interest such as between 20 Hz and 15 kHz. Furthermore, practical volume velocity sources may have dimensions that make these impractical or impossible to arrange at certain ones of the assumed source positions.
An alternative methodology for determination of individual noise sound contributions in automotive applications is the so-called masking method. According to the latter masking methodology, individual or several noise sources are masked or insulted using heavy material such as lead, or extra mufflers for intake and exhaust outlet to eliminate noise sound contributions from these noise sources while measuring noise sound contribution of the target noise source. As such the contribution of each of the unmasked noise source can be assessed in theory. However, this methodology gives very crude results and is overall impractical to assess the multitude of separate noise sources present in a complex vehicle construction like a car, truck or bus etc. The Internoise paper by Jakob Putner ET AL: “Operational transfer path analysis predicting contributions to the vehicle interior noise for different excitations from the same sound source”, InterNoise 2012, 22 Aug. 2012 (2012-08-22), XP055090134, New York, USA discloses a method of determining individual noise sound contributions from various physical noise sources such as engine, gearbox and exhaust, at a reference position inside a vehicle cabin. The noise source contributions are measured at different operating conditions of the vehicle. The measurement methodology is referred to as Operational Transfer Path analysis (OTPA). A linearized transfer function between a selected noise source and a selected reference position is calculated.
The technical paper by Junji Yoshida ET AL “2013 The Japan Society of Mechanical Engineers Contribution Analysis for Vehicle Interior Noise Using Only Response Signals”, 25 Apr. 2013 (2813-04-25), XP055889844 discloses a method of determining noise sound contributions from individual noise sources, such as engine and wind noise, to a mixed response signal (target signal) measured inside the interior of a vehicle. The proposed methodology utilizes only the mixed response signals and not any individual recordings of the noise source signals. Frequency domain ICA is applied to the mixed response signal to separate the contributions from the individual noise sources.